1. Field of the Invention
The present invention relates to a new light source having an artificially formed color center (hereinafter referred to as an "artificial color-center light source") in which an atom is removed from the surface of an ionic crystal so as to form a defect artificially and to use the same as a color center.
2. Description of the Related Art
A color center laser is a known light source that operates on the basis of random defects formed in an ionic crystal.
A structure in which a single atom is trapped in an optical resonator to provide a light source is disclosed in a paper entitled "Microlaser: A laser with one atom in an optical resonator" reported in a recent Physical Review Letter.
In the light source, a barium atom beam is passed through the resonator and is optically excited so as to function as a gain medium. Statically, a single barium atom is present in the resonator. Therefore, the author of the paper named the apparatus "single atom optical resonator."
Microlight sources developed thus far are of a type that utilizes transition between energy bands of a semiconductor material and/or structure. Especially, semiconductor quantum-well devices utilizing quantum confined structures and like devices have been used.
However, this invention, in which a single lattice defect is formed in a resonator with atomic level accuracy, enables generation of light having a constant wavelength in a more stable manner as compared with conventional light sources. Moreover, the characteristics of the light source are determined by well defined energy levels of the single defect and an interaction between a field of photons and a coupled vacuum state with the resonator. Therefore, this structure facilitates handling of a light source in quantum optics. It is also important that the basic structure of the light source can be constructed through use of a homogeneous material of a single kind.
Of course, since the size of the structure is at the atomic level, in the future, the light source will be usable as a microlight source in optical communication as well as in information transmission of an optical computer. In contrast to conventional light sources which mainly utilize transition between energy bands of a semiconductor material/structure, the present invention utilizes energy transition between levels that are determined by the characteristics of an atom itself, which is a more basic element. And, in the present invention, due to electrical confinement, the transition probability becomes higher than that in a bulk material.
With recent progress in developing a technique of manipulating a single atom through use of a scanning tunnel microscope (STM), it has become possible to manipulate the spatial position of each individual atom. A paper entitled "Scanning Tunneling Microscope Fabrication of Atomic Scale Memory on a Silicon Surface" (Dehuan Huang, et al., J. J. A. P. 33, 190 (1994)) discloses a technique of arbitrarily removing and inserting silicon atoms that form a crystal structure on a silicon substrate through use of an ultra-high vacuum STM. Through use of this technique, an attempt has been made to reproduce a defect at the atomic level and to provide an atomic-level memory utilizing the presence/absence of an atom.
FIGS. 1(a) and 1(b) show artificial lattice defects that were formed through use of the conventional STM. In FIGS. 1(a) and 1(b), the images of silicon atoms are shown as white patterns, while the sites from which silicon atoms were removed are depicted by black patterns. FIG. 1(a) shows an image of an atomic plane before removal of atoms, while FIG. 1(b) shows an image of an atomic plane after atoms at locations indicated by black "+" marks in FIG. 1(a) were removed by the STM. These images clearly show that it is possible to remove an atom from any position on a silicon atom plane.
A lattice defect in an ionic crystal (e.g., LiF, NaCl, NaF, KI) is known to exhibit an optical transition property (usually called a "color center") which is represented by a quantum effect in a quantum box. This characteristic has been used in, for example, a color center laser. Application to a color center laser is disclosed in, for example, "Laser Handbook" (Ohm Corporation). Quantum effect of a color center is disclosed in, for example, "Defects in Crystalline Solids" (written by B. Henderson).
A site from which an atom has been removed (i.e., a defect) serves as an energy well formed in the potential of surrounding atoms. Since the size of the well is smaller than the wavelength of an electron, a quantum confinement effect takes place and quantizes energy levels to discrete values; i.e., quantization levels. Especially, when a negative ion is removed from an ionic crystal so that a defect is formed therein, electrons are attracted by a positive charge generated at the defect and are trapped in the vicinity of the defect. When a trapped electron falls to a lower energy level created by the well, the balance energy is radiated in the form of an electromagnetic wave, which is the light emission process of a color center.
As described above, when only negative ions are removed with atomic-level accuracy, quantized energy levels at which electrons are restrained are established, so that stable light having a narrow spectral width can be generated while avoiding the conventional drawback in which a light source has a broad emission spectra due to inhomogeneity of the randomly created defects.